Cooling can occur in electric machines arranged as power transformers by emitting the heat generated during the operation of a dry-type transformer directly to an ambient environment.
In electric machines, for example, high power electric machines arranged as compact power transformers, additional cooling measures can be used in addition to cooling by convection.
As a result, cooling channels for the cooling of a compact power transformer can be provided in known housing concepts. The cooling can occur by free convection, which is natural cooling.
This can be insufficient in the case of higher transformer outputs, so that the design of the housing can be adjusted or an additional separate cooling system with forced cooling can be provided.
In known cooling arrangements, which can apply to power transformers arranged as dry-type transformers, a dry-type transformer can be surrounded by a housing which provides improved cooling of the transformer by purposeful flow about its windings with cooling medium which is cooled, for example, with air-to-air coolers or air-to-water coolers.
An enclosure for a dry-type transformer is known from DE 19812243 wherein ambient air can flow all around through the enclosure, with the air cooling off and being wetted during the through-flow of an outer wettable jacket which is wetted with water either continuously or in a load-dependent fashion. The cooled and humidified air is guided in such a way that it can continuously cool the solid body surfaces heated by the high-voltage windings. The air can enter from below into the cooling channels of the individual transformer legs and can be sucked off upon exiting from the cooling channels and guided away.
This known cooling principle is based on the main components of the air, which are nitrogen and oxygen, being very bad heat carriers and virtually permeable for thermal radiation. A number of gases, especially carbon dioxide and water vapour, on the other hand, absorb radiation heat and radiate the same.
A high content of water vapor in the unsaturated state in the cooling air of a transformer can improve the thermal absorption capacity of the same. It is further known that water has a very high heat of evaporation. It is approx. 627 Wh/kg at normal pressure, so that the use of water can provide favorable heat absorption in that the air used for cooling the transformer is enriched with water. The water is allowed to evaporate as a result of the heat absorbed from the transformer and heat can be removed from the transformer.
For the regular addition of water to air a closed cooling system is desirable in order to limit the consumption of water.
On the other hand, it is desirable to newly configure and mount the respective cooling system for each transformer in the development of a transformer housing with forced cooling. The possibility of natural cooling by convection is lost in this case, which means that such transformers can then only be cooled in a forced way.
Natural cooling within and outside of the housing can be sufficient under constant ambient conditions up to a certain size of the housing in order to remove a specific amount of dissipation. In order to remove dissipation that goes beyond this or to ensure compensation for increased ambient temperature, further measures for cooling are involved.